1. Field of the Invention
The present invention relates generally to a method and system for controlling tension to be exerted on a metal strip in a continuous annealing furnace for preventing the metal strip from meandering in the annealing furnace. More specifically, the invention relates to a technique of controlling tension on the metal strip in the continuous annealing furnace relative to thermal crown magnitude for suppressing meandering of the metal strip. The invention also relates to a tension control for annealing a very thin and flexible low-carbon metal strip or extra low-carbon steel strip.
2. Description of the Background Art
In general, continuous annealing of a series of metal strip is performed in a continuous annealing furnace which defines the path of the metal strip by means of a vertically offset plurality of hearth rolls. The series of metal strip passes a substantially long path in the annealing furnace with wrapping over the hearth rolls. When the metal strip offsets with respect to the center of the defined path, the offset magnitude can be amplified by the substantial length of the metal strip to cause substantial meandering of the metal strip in the furnace. This can cause breakage of the metal strip by contacting with the peripheral wall of the annealing furnace. Therefore, meandering of the metal strip has to be suppressed for stably performing a continuous annealing operation.
For this purpose, a crown is provided on the hearth rolls for automatically suppressing meandering of the metal strip by exerting a centering force. As a known technique, a taper crown which gradually reduces the diameter of the roll at both ends to a formed tapered profile at an end portion, and a round crown which reduces the diameter of the roll for forming a rounded profile at both ends of the roll, have been conventionally used. In these two types of crowns, a taper crown has been widely used because of ease of production and better centering performance. In the practical operation, the flexible metal strip tightly wraps on the periphery of the hearth roll. At this condition, a force arranged to transversely shift the metal strip toward the section having the larger diameter is exerted. This transverse force serves as a centering force for centering the metal strip. The magnitude of the centering force is variable depending upon the magnitude of tension exerted on the metal strip. Namely, the magnitude of the centering force to be exerted on the metal strip increases with increasing the magnitude of the tension to be exerted on the metal strip. This means that greater magnitude of tension force to be exerted on the metal strip may exhibit better strip centering performance. However, the tension to be exerted on the metal strip is limited in view of the strength of the metal strip to be annealed so that breakage or deformation of the metal strip may not occur.
On the other hand, by the centering force, the metal strip is centered to constantly pass the central portion of the hearth roll to form a cover for the central portion of the roll. Therefore, while wrapped by the metal strip in a heating zone and soaking zone in the annealing furnace, the central portion to the hearth roll may not be subject to the heat in the furnace. On the other hand, the transverse end sections where the crown is provided are constantly subject to the heat in the furnace. As a result, a difference of thermal expansion in the radial direction between the central portion and the end portions occurs. In this case, the thermal expansion at the end portions becomes much greater than that in the central portion. This difference of thermal expansion causes reduction of the magnitude of crown on the hearth roll to reduce the centering force to be exerted on the metal strip. When the difference of the thermal expansion becomes significant, a substantial change of roll crown is caused to result in meandering or heat buckling of the metal strip.
On the other hand, in the cooling zone, the central portion is constantly subject to high temperature heat transmitted from the metal strip to differentiate the temperature between the central portion and the end portion. Because of higher temperature at the central portion, thermal crown tends to be increased.
When the line speed of the metal strip in the annealing furnace changes significantly, the temperature at the central portion changes more quickly than the change at the end portion. Therefore, temperature distribution at the central portion and end portion of the hearth roll changes significantly to cause variation of the roll crown. When the crown magnitude is excessively increased by change of the line speed, heat buckling tends to occur. On the other hand, when crown magnitude is decreased by change of the line speed, meandering of the metal strip tends to occur.
For preventing the aforementioned problem, there have been proposed heating and/or cooling the hearth roll, or adjusting the crown magnitude by means of roll bending device. Such prior proposals have been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa 57-177980 and the Japanese Utility Model First Publication (Jikkai) Showa 55-172859. However, in these cases, a substantial number of hearth rolls has to be adjusted for crown magnitude independently of each other. Therefore, in viewpoint of the cost, such prior proposal is not practically applicable for the actually working annealing furnace.
On the other hand, in annealing of substantially thin and flexible extra low carbon metal strip, prevention of the meandering and heat buckle is especially important. Particularly, in case of annealing metal strip such as a soft-temper tin plate, tin-free strip (TFS), which has a carbon content lower than or equal to 100 ppm. The crown of the hearth roll and the tension to be exerted on the strip has to be quite delicately controlled so as not to cause meandering and heat buckle.